Variable speed drive interpolation system for automatic machine control



1962 R. w. TRIPP 3,058,657

VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed NOV. 15, 1956 15 Sheets-Sheet 1 ROBERT W. TRIPP,

IN VEN TOR.

A TTORNEY.

Oct. 16, 1962 R. w. TRIPP 3,058,657 VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 2 ROBERT W TRIPP,

INVEN TOR.

A TTORA/EK Oct 1962 R. w. TRIPP 3,058,657

VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 3 A TTORNEK Oct. 16, 1962 R. w. TRIPP 3,058,657

VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 4 AAX i l I D i-"AY l AAY mm Hun) mx f g,za, )fgoyba P l AX D )llfl-Jd.

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+Ror-R A XR ROBERT W. TRIPP,

IN VEN TOR.

BY, MM

ATTORNEK Oct. 16, 1962 R. w. TRIPP 3,058,657

VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 5 DIGITAL COMPUTER FIG. IO FIG. II FIG. I2 FIG. I3 FIG. I4 FIG.I5 FIG. l6

ROBERT W. TRIPP,

INVENTOR.

A T TORNE K Oct. 16, 1962 R w TRIPP VARIABLE SPEED LSRIV'E INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 6 300\ son 3021 l 3031 304 STOP 8 REE 285%3L r38 I:MULTIPLIERS 0! Z D 1 N 305 295 AX TAPE AY a READER DISTRIBUTOR sT guGE RELAYS is Z 306 6 A 2 X X j" W TRANSLATOR Z FEED RATE 8 Y /IO 222 FEED RATE REE CONVERTER 93a 309 I 9| 92 mo 5n ROBERT W. TRIPP, x 1 VARIABLE GAIN INVENTOR.

CONTROL 59' RoM TAPE DATA y W ATTORNEY.

Oct. 16, 1962 R. w. TRIPP 3,058,657

VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 7 MUL IPLI S SERVO NULL RELAY GAIN CONTROL SLOPE ANGLE REE SERVO MOTOR ABSOLUTE VALUE OF SLOPE ANGLE OF CURVE 3'3 AXAY STORAGE RELAYS REF. 3O

A|Y+YR AMPLIFIER REE A X+X AMPLIFIER REF.

FEED SERVO 308 308 AMPLIFIER ROBERT W. TRIPP,

INVENTOR.

,4 TTORNEK Oct. 16, 1962 R. w. TRIPP 3,058,657

VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 8 ROBERT w. TRIPP,

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ATTOR/VEK Oct. 16, 1962 Original Filed Nov. 15, 1956 R. VARIABLE SPEED DRIVE INTERPOLATION SYSTEM W TRIPP r3|4 3 4 324 r3l6 3'67 INTERPOLATOR 1 7*" SEQUENCE 302 AX 3. 2 323 CONTROL v IIIHIIHII l 1 SOLENOID ACTUATORS 392 WWW- 211 SPEED UP l BINARY GEAR MECHANISM X BRAKE a CLUTCH WW @WWE Q BINARY GEAR MECHANISM AY 1 I 2Z| SPEED UP SOLENOID ACTUATORS 49 l mm AY L 1 1 53 E320 ROBERT W. TRIPP,

IN VEN TOR.

ATTORNEK Oct. 16, 1962 W TR PP R..I VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 10 XPOSITION -,rseavo 325 MOTOR RESOLVERS "'15? RESOLVERS L "T E T T XI+5AX 44 e4 63 x I l3 FEED RATE r43 313 SERVO TAcHoME'n-:R

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ROBERT w. TRIPP,

INVENTOR.

ATTORNEK Oct. 16, 1962 R. w. TRIPP 3,058,657 VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Ongmal Filed Nov. 15, 1956 15 Sheets-Sheet 11 Y MOTOR L ssnvo Y MOTOR 3" ROBERT W. TRIPP,

INVEN TOR.

Jig. 16'. BY 7 A TTORNEK Oct. 16, 1962 R. w. TRIPP VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 12 Own IOFOE ROBERT w. TRIPP,

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4 7' TORNE K Oct. 16, R w TRIPP VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 1 3 ROBERT W. TRIPP,

INVENTOR.

A 7' TOPNE V.

1962 R. w. TRIPP VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL Original Filed Nov. 15, 1956 15 Sheets-Sheet 14 OUTPUT ROBERT w. TRIPP,

INVENTOR.

ATTORNEK 06L 1962 w TRIPP 3,058,657

R. VARIABLE SPEED DRIVE INTERPOLATION SYSTEM FOR AUTOMATIC MACHINE CONTROL original Filed Nov. 15, 1956 15 Sheets-Sheet 15 m o o ll ul & 5 0 I o O O. a z in i a Q] m 2 R 9 2 (0 L0 qw O (SEIHONI NI) SiNlOd NBEIMiI-IB BONVLSICI ROBERT W. TRIPP,

INVENTOR.

BYW.

A T TORNEK United States 5 Claims. 01. 235-154 The invention relates to a variable speed drive interpolation system for automatic machine control which will accept input data in digital form and control the motion of a tool relatively to a work piece along a straight path, or along a continuous curve instead of generating a straight line segmental approximations as heretofore proposed, for example in the =M.I.T. milling machine control. The tool may be either the cutting tool of a milling machine, lathe, profiler or the like, or it may be the scriber or stylus of a drafting or engraving machine for generating curves.

In US. application, S. N. 557,035 filed I an. 3, 1956 for Automatic Machine-Control Method and System, now Patent 2,875,390 dated February 24, 1959, hereafter referred to as Case 3 there is disclosed and claimed an automatic machine tool control method and system which will also accept digital input data for the purpose above mentioned. Case 3 and other patent applications referred to herein are assigned to the assignee of the present application.

The objects of the present invention include the features of an improved variable speed drive for reducing the amount of input data required, eliminating much of the effort required for programming the portion of the computation carried out on a digital computer, and resolving the instruction to the machine elements into a plurality of grades of increments of which the fine increment is preferably handled by a highly accurate positionmeasuring transformer such as the Inductosyn as described in further detail later.

Case 3 and several other pending applications referred to later describe and claim the generation of a complex curve by what is essentially extrapolation, in that the various inputs namely, the coordinates of the points and slope, curvature and rate of change of curvature are integrated to direct the path towards the next point on the curve to be generated. This approach involves digitalto-analog conversion of input data into a shaft rotation, resolving the angular position or movement of the shaft into co-function values having the relation of sine and cosine of the shaft angle and integrating the feed rate of the machine drives along the X and Y axes with such sine and cosine values respectively whereby the machine elements are controlled relatively to each other in a manner defining the straight line or complex curve designated in the input. Depending upon the accuracy of the constants and of the integration, the line or curve thus generated may or may not hit the next point exactly. Thus there is need to provide for checking and correcting as described and claimed in US. application S.N. 563,125, now abandoned, filed Feb. 2, 1956 for Automatic Digital Machine Tool Control, hereinafter referred to as Case 4.

As compared to Cases 3 and 4, an object of the invention is to simplify the apparatus required, avoid inaccuracies due to use of mechanical devices such as balldisk-cylinder integrators for performing mathematical operations, avoid the necessity for checking the correctness of the tool path, and provide apparatus wherein the correctness of the tool path is inherently checked, while Registered trademark.

atent "ice generating or machining a continuous curve. This is accomplished by generating the curve by an interpolation method in which the curve is fitted between point pairs and goes through each point without errors of integration. The accuracy of the approximation to a spline fit depends upon the separation of the points and upon the nature of the curve as explained in further detail later. The amount of data required according to the invention can be reduced by selecting the distance between data points with due regard to the required curve shape and the permissible error. The use of the variable speed drive of this invention also makes it possible to maintain a constant cutting speed even though each cycle of the feed rate includes point pairs having different separations.

It is possible to obtain the required machine input data with certain relatively simple arithmetical operations. The specification develops the required mathematical equations. The only original data needed for the evaluation are the X and Y coordinates of a suitable number of points. When the equation of the curve is known, it is a very simple matter to compute the necessary input data to the machine, as well be hereinafter explained.

A particular advantage of the invention is that the machine can be stopped at any point of its sequence and be started again without loss of accuracy.

According to the invention, a curve is divided into segments, and the coordinates of each segment are taken as the sum of the coordinates of points along the chord between the point pairs for each segment and the distance from the chord to the curve, measured perpendicular to the chord. The invention is useful for continuously computing this sum in analog form from data of the segment and the chord and data of the required interpolation constants. A further object of the invention is to provide a variable speed drive useful in connection with the steps of continuously computing from such data the lengths of the perpendicular from the chord to the curve, resolving such perpendicular values into their components along the X and Y axes, adding such components to the corresponding components of the chord along such axes, and controlling the feed rates of the machine elements along those axes accordingly.

While the variable speed drive of this invention may be used for generating a straight line, or a succession of straight lines having difierent slopes, the invention is particularly useful for the generation of complex curves by generating successive curved segments. This involves the chord-to-tangent angle and the length of the chord for each segment and for the generation of continuous curves, it also involves preparing the instruction for the next segment while maintaining the instruction for a given segment with a quick shift from one instruction to the next. It is a particular object of the present invention to provide a variable speed drive useful for those purposes. Further features of the invention are to provide for adjusting the position of the required curve before starting, to provide for a separate introduction of cutter offset instruction, to provide a constant cutting rate independent of the length of each successive segment of the curve to be cut. A further feature is to provide for the generation of corners including means for stopping the feed rate drive until the new slope angle is established, while maintaining the cutter radius instruction active so that at the corner the machine will describe an arcuate path having a radius equal to the cutter radius whereby the cutter always remains in contact with the corner being cut. Provision is also made for zero offset as described and claimed in application S.N. 638,722, now Patent No. 2,950,427, filed Feb. 7, 1957 for Zero Offset for Machine Tool Control. In general, these objects and features are accomplished as follows.

The curve is divided into a plurality of successive segments the length of these segments being chosen with due regard for the required accuracy as explained later.

Slope angle.-The slope of the path depends on the ratio of the feed rates along the X and Y coordinate axes. This ratio is established by extending each of the X and Y drives through a variable speed ratio device, here illustrated as an improved digital gear device. The digital gear device disclosed and claimed herein is binary and, as to its generic aspects, is described and claimed in Case 3.

Distance.-The cord of each successive segment of the curve is obtained as follows. The two gear ratio units referred to above have an input shaft which is driven through one revolution or an integral number of revolutions for each segment, corresponding to a cycle of operation, and the gear ratio is changed only at the end of a cycle. This results in the X and Y output drive shafts making a number of revolutions or parts of revolutions proportional to the digital inputs. Thus the total angular travel of the two output shafts, for the X and Y axes, is proportional to the distance along those axes required to generate the chord of the segment.

Instruction shift.Storage relays are provided for transfer of the input data into the gear ratio units during the cycle, without any pause in the motion of the output shafts for continuous curves. Also, each of the gear devices is constructed and arranged to maintain a given ratio while being urged to an alternate position corresponding to a change in the ratio, such change being prevented, however, until the end of the cycle, where the change is quickly effected. During the cycle, the analogcomputer operates in synchronism with and may be driven by the input shaft for the gear units so that the computation of the curve data (X and Y components of the perpendicular distance from the chord to the curve), proceeds in space phase relation with the X and Y feed rate instructions which determine the length and slope of the chord.

Psiti0n.This is added to the instruction generated by the analog computer equipment so that the machine may generate a curve at any desired point within its capacity.

Cutter ofiset.Pr0vision is made for computing the absolute value of the slope angle of the curve from the input values of the increments AX and AY, an adjustable tool radius input R being provided to resolve the tool radius R into the increments X and Y for addition to the other instructions pertinent to the X and Y axes. This is broadly described and claimed in U.S. application S.N. 561,769 filed January 27, 1956 for Tool Radius Correction Computer as applied to two axes, and is described and claimed as applied to three axes in S.N. 608, 357 filed Sept. 6, 1956 for Three Dimensional Tool Radius Correction Computer. Use of this feature permits various cutters to be employed without altering the input program.

Constant cutting rate-As successive cycles may involve chords of diiferent lengths, provision is made for operating the input drive to the gear ratio units at a speed inversely proportional to the length of the chord whereby a speed inversely proportional to the length of the chord whereby a substantially constant cutting rate is obtained.

Corner routine.-As described above, provision is made for reducing the feed rate to zero at a corner until the instruction of a new slope angle is set in, while maintaining the cutter radius instruction active so that the cutter always remains in contact with the corner being cut.

Zero ofjset.Provision is made for locating the program zero as described with respect to the machine coordinates so that the part may be programmed in advance and the part located on the machine subsequently. The difference between the machine coordinate zero and the part coordinate zero is zero ofiset. This is described and claimed in US. application S.N. 638,722,

4- noW Patent No. 2,950,427, filed Feb. 7, 1957 for Zero Offset for Machine Tool Control.

Sequence.Concerning the sequence of operation of the digital gear device in relationto the program advance, the objects of the invention are to hold the inputs to the differentials in the selected state from one command to another, change from one state to another at exactly the position desired, hold the digital input for a short time until the equipment is ready to accept it, hold the inputs energized independently of the storage circuits, utilize the storage circuits to hold the next data while current data is being used, and to provide a circuit which makes it possible to read the punched card or tape at a relatively slow rate and during times when the previous information is being held on the operating coils, while making it possible to change the state of the operating coils, and obtain the shaft speed called for by the input, very rapidly and at an accurately chosen time or under accurately chosen conditions.

In prior control systems referred to above, it has been customary to machine successive straight line segments to produce an approximation of the desired surface. A more satisfactory machined part and a very large reduction in the amount of the required input data, as well as increased flexibility and accuracy are obtained according to the present invention wherein the inputs include differences of position of points on the surface and interpolation constants for the surface to be cut, whereby the method and apparatus of the present invention are capable of machining continuous curves instead of straight line segmental approximations of the curve or surface to be cut.

In accordance with the present invention, each successive segment of a curve is generated from the values of (a) the diiferences between the coordinates of the end points of the segment, (b) the angle between the chord and the tangent at the cutting point, and (c) the departure H of the curve from the chord, measured perpendicularly to the chord from the cutting point on the curve. It is shown that the value of H is given by the equation:

and that the value of the angle A is given by the equation:

In the above Equations 1 and 2, B =starting interpolation angle of segment a =ending interpolation angle of segment D=chord distance between adjacent data points. S=distance measured along the chord to a perpendicular from the chord to the cutting point on the curve.

It is further shown that instantaneous values of the coordinates of points X, Y, on the path to be followed by the center of the cutter are given by the following equations which appear later as Equations 62 and 63:

X =abscissa of hte first data point N X :diiference in abscissae between adjacent data points X =X component of cutter radius Y =ordinate of the first data point Y=difference in in ordinates between adjacent data points Y =Y component of cutter radius It is also shown that the value of tan A as given in Eq. 2 is taken into account in computing the cutting offset components X and Y A further feature of the invention is to provide a method of computing the necessary input values of a, [-3, AX and AY in digital form and continuously computing signal valeus of H, (Eq. 1) and D and also tan x, (Eq. 2) and from these signal values to continuously compute the terms included in Equations 62 and 63 and add the terms for each equation as indicated, and control the feed rate of the machine elements on coordinate X and Y axes accordingly.

It is shown that in the case where the equation of the curve is known, the values of a and 5 are readily determined, whereas, if the equation of the curve is not known, the values of a and [3 may be computed by analogy to a spline fit, from data of the points of preceding and succeeding segments of the curve.

The above objects are accomplished by employing a precision position measuring transformer to attain high precision, and by employing analog computers to provide continuous data of position, to drive servos which position the machine element relatively to the work piece to be cut.

While the invention will be described with reference to two orthogonal axes, referred to as the X and Y axes, with the Z axis used for positional control only, it will be apparent that the interpolation method and means herein described may be extended to apply also to a third axes Z at right angles to the plane of the X and Y axes.

There are several known mathematical interpolation methods by which an equation, or a series of equations, can be obtained, which will closely approximate any desired curve. In the preferred mathematical method herein described, the equations of the curve itself are not computed or required. Instead, the mathematical theory of stress and strain is employed to compute selected parameters of a spline fit to the required curve, or a very close approximation to such spline fit.

The selected parameters are computed for successive pairs of points along the desired curve as given, or taken I suificiently close together to insure the required accuracy of approximation to the desired curve. The computation for each pair of points is based upon the relative locations of prior and subsequent points along the curve. So far as is known, this mathematical method of producing a close approximation to a desired curve has previously not been disclosed.

The equations which are herein developed and employed to give effect to the location of prior and subsequent points in determining the path to be followed between each successive pair of points, are based upon the use of two prior and two subsequent points. It is to be understood, however, that the mathematical method is equally applicable to single prior and subsequent points or to three or more prior and subsequent points, and that the invention is therefore not limited to any particular number of prior and subsequent points.

The selected parameters to be usedare computed on a digital computer which may be of any suitable type, and which is not claimed as part of the invention. These parameters are recorded by the digital-computer in any convenient form, such as punched cards or punched paper tape.

The computed parameters are employed in a combination'of electrical, electronic and mechanical compo- 6 nents to be described, to cause the cutter or other tool of a milling machine or the like to reproduce the desired curve upon any desired number of work pieces.

In the illustrative embodiment herein described, the X and Y components of the perpendicular distance from the chord between any two points to successive points on the interpolated curve are computed in analog form and added to the successive X and Y components of the chord, to guide the cutter or other tool along a path which will reproduce the interpolated curve.

The present application is a division of SN. 622,397 filed November 15, 1956 by Robert W. Tripp for Inter-polating Method and System for Automatic Machine Tool Contro which discloses and claims the machine tool features of the present case. The binary gear device disclosed herein is disclosed and claimed in Ser. No. 683,402 filed September 11, 1957 for Binary Gear Device, now Patent 2,902,887 dated September 8, 1959. The computing method and system disclosed herein is disclosed and claimed in Ser. No. 683,403 filed September 11, 1957 for Interpolation Computing Method and System for Automatic Tool Control. The last two mentioned applications are further divisions of Ser. No. 622,397.

For further details of the invention reference may be made to the drawings wherein FIGS. 1, 2, 3, 4a to 4e and FIG. 5 are schematic figures illustrating a curve and its components involved in mathematical equations given later in connection with computing the segment of the curve from certain parameters of the curve, FIGS. 4a to 4e, being useful in connection with computing the starting and ending chord-to-tangent angles of the curve where the curve function is not known. FIG. 4a illustrates a spline fit through 6 points. FIGS. 4b to 4e illustrate the 3-point spline components of the curve in FIG. 4a.

FIG. 6a is a schematic diagram of a portion of the system in FIG. 12, illustrating a resolver with its inputs and outputs for computing the slope angle between the X axis and the tangent at the. cutting point on the curve, FIG. 6b being the corresponding geometrical diagram.

FIG. 7a is a schematic diagram of another portion of the resolver referred to above, illustrating the inputs AX and AY for computing the chord length D, FIG. 7b being the corresponding geometrical diagram.

FIG. 8a is a schematic diagram of another portion of the resolver device of FIGS. 6a and 7a illustrating the tool radius input to be resolved into components along the X and Y axes, FIG. 8b being the corresponding geometrical diagram.

FIG. 9 is a block diagram illustrating how FIGS. 10 to 16 are arranged edge-to-edge to illustrate a complete system capable of operating in accordance with the method the present invention, the input of FIG. 10 operating through the various computers and controls illustrated in FIGS. 11 to 15, to control the machine elements illustrated in FIG. 16.

FIG. 17 is an enlarged view of the Zero offset control illustrated schematically in FIG. 16, FIG. 17 illustrating this item for the X motor, a similar control, not shown, being provided for the Y motor.

FIG. 18 is a view, partly in section, which may be considered either a plan view or a side view in elevation of the gear mechanism and associated parts illustrated in FIG. 14 for the X axis, a similar gear mechanism with its associated parts not shown being also provided as shown schematically in the Y axis in FIG. 14.

FIG. 19 is an enlarged sectional view of the gear device of FIG. 18, the section being taken on line 1919 of FIG. 20, looking in the direction of the arrows, and illustrating the controls for reversing the drive.

FIG. 20 is a section taken on line 2020 of FIG. 19.

FIG. 21 is a section taken on the broken line 2121 of FIG. 19 and shows the gear 159 which meshes With the gears 134 and 135.

FIG. 22 is a diagram of the linear digital-to-analog converter and multiplier employed in the systems of FIGS. 10 to 16.

FIG. 23 is a graph illustrating the relation of the distance between data points to the departure of the generated curve from a circular arc.

FIG. 24 is an enlarged sectional View, with parts broken away, illustrating the clutch or detent arm for each of the ten binary gear drives 100 to 109 in FIG. 18.

The following is a list of the symbols used in this specification, with their definitions; as illustrated in FIGS. 1 to 8. X=instantaneous value of the abscissa of the cutter center along the machine X axis Y=instantaneous value of the ordinate of the cutter center along the machine Y axis X =abscissa along X axis of data point N Y =ordinate along Y axis of data point N X =abscissa of the cutting point along X axis Y =ordinate of the cutting point along Y axis X =X component of cutter radius Y =Y component of cutter radius AX=difference between the abscissae of adjacent data points AY= difierence between the ordinates of adjacent data points A X=X component of H (see H below) A Y=Y component of H (see H below) H=departure of the curve from the chord D, measured perpendicular to D from the point P on the curve. H'=deflection of a beam anchored at point 1 and bent to pass through point 2. (See FIG. 2.)

D=chord distance between adjacent data points C=constant S distance measured along the chord D R=cutter radius a=slop angle between the chord D and the X axis x angle between the chord and the tangent at the cutting point P =the angle between two successive chords =slope angle of the tangent at P ,B the angle between the chord of a curve segment N, N +1 and the tangent to the curve segment at N,

or, chord-to-tangent angle. a =the angle between the chord of a curve segment N 1, N and the tangent to the curve segment at K, or,

chord-to-tangent angle.

[3:5,, as applied to Equations 1 and 2. (See FIG. 3.) 0t=OL For 4, fi l3 =fi 3 and OL:0t =lXb4 AAX: (tan MAY AAY=(tan MAX See FIG. 6.

Sign Conventions The sign conventions employed in the following discussion are:

(1) Angles are positive when measured counter-clockwise.

(2) An angle between a chord and a tangent is measured from the chord to the tangent.

(3) An angle between two chords is measured from the extension of the first chord to the second chord.

(4) Distances along chord are taken as positive in the direction of motion. Distances normal to chords are taken as positive to the left of the direction of motion.

(5) Lengths of chords are taken as positive.

Mathematics of the Interpolation Meth d Consideration will now be given to the mathematics of the interpolation method, first for the general case where the equation of the curve is not known, and then for the case where the equation of the curve is given.

Equation of the curve not kn0wn.-The general case is the one in which the equation of the curve is not known. In this case, the digital computer 2', FIG. 10, computes the lengths D of the chords and the angles 6 from the 8 values of X and Y between the chords. Using Equations 35, 36, 37 and 38 to find values of C and C it then computes the required values of the chord-to-tangent angles at and ,8 from Equations 33 and 34. These equations are explained later.

Equations 1 and 2 are those of the required parameters of a close approximation to the curve which would be produced by a spline, or a uniform flexible strip which is caused to pass through the given points on the curve. The use of such a spline or strip is well-known in the drafting and layout arts. The validity of the equations may be demonstrated in the following way: If a beam 1, 2 (FIG. 2), fixed at the point 2, is bent by a force normal to the beam at point 1 sothat the beam after bending passes through the point 1, its behaviour may be analyzed in the following way: there is set up a bending moment which varies along the beam, its magnitude at any point P being proportional to the distance, S, from the point 1. This is a well-known principle, fundamental in the theory of stress and strain, and presented in any treatise on strength of materials. In FIG. 2, and, indeed, in the usual treatment of beams, it is considered immaterial to the discussion whether the distances involved are measured along the tangent 1, A, along the chord 1, 2 or along the bent beam 1, P, 2 itself, since the angle A, 1, 2 is considered to be so small that the relationship angle=sine=tangent is a very close approximation. In the operation of the invention herein described, it is necessary that the points along the curve to be out be taken sufiiciently close together that this relationship is true without sensible error. This is not a tight restriction since, for reasons of accuracy, points would normally be taken sufi'iciently close together that this is the case.

The bending moment of the beam which is proportional in magnitude to the distance S along the beam (or along the tangent, or the chord), causes a change in the slope of the beam such that the rate of change of slope is proportional to said bending moment. From the principles of analytical geometry it is known that the slope of a curve H=f(S) is given by the first derivative of H with respect to S, and the rate of change of slope is given by the second derivative of H with respect to S, or in common nomenclature, and with the notation of FIG. 2:

H"=f(S) dH (is =f( p ;=f(8) =rate of change of slope (8) But it is known that the rate of change of slope is proportional to S, hence we may write mation. Substituting these values in Equation 11 gives k= 12 We are interested, however, in the value of H, or departure of the curve from the chord. Referring to FIG. 2 and using the small-angle approximation:

and the slope of the curve with respect to the chord may be obtained by taking the derivative of (17) with respect to S, obtaining Now, at the point (2), S=D, and substitution in (18) gives where a is the chord-to-tangent angle at the point 2.

In the case of the present invention, the beam in question' (the assumed spline) is not fixed at either end of a given chord, so that the above analysis is insufficient to completely describe the case. The case may be described, however, to a close degree of approximation by considering a set of six (6) points of constraint, two to the left of and two to the right of the chord segment being considered. If this is done, the condition shown in FIGS. 4a to 4e exists, Where the symbolism is consistent with that adopted in the above discussion.

In accordance with the principal of superposition, there is a unique set of four 3-point beams which determines the shape of the spline through six points. As will be explained in more detail presently, the chord-to-tangent angles a and [3,, of the 3-point beams are proportional to the chord lengths, and depend in magnitude upon the angles between successive chords, since, at each point, the sum of the on components plus the sum of the [3 components is equal to the chord-to-chord angle 0. (See Equation 28.) The sum of a and 6,, at the center of each 3-point is an angle 0, which in general is not equal to 0, and need not be determined.

FIGS. 4b to 4e show the component 3-point splines used to compute the 6-point spline 4a. The curvature is zero at the end points of the four 3-point beams. Hence, at either side of the mid point of each 3-point beam, the chord-to-tangent angle is twice that the end of the beam, but is of opposite sign. (See Equation 19.)

The actual values of the chord-to-tangent angles at the chord ends are the algebraic sums of the component values illustrated in FIGS. 4b to 4e, that is to say fiN=BaN+bm n=m+an From the theory of beam fiexure, it may be shown that where C is a proportionality constant. If 11, is proportional to D as is indicated in Equation 23, then it can be stated that oc =D C and the equations for the component angles are as follows:

Now, the angles between the chords can be seen by inspection to be equal to the sum of the actual angles e and [3 at a given point, and since, by the principle of superposition, the angles rm and [3 at a point are the sums of the respective components of these angles -i-a and fl -l-fi we may write, for the angles between the two chords at the points 3 where the algebraic signs have been appropriately chosen for the directions in which the angles are indicated. Now, from Equation 19 And, substituting in 28 from Equations 24, 25, 26, 27, 29, and 30 we obtain and the values of 3, and 0%,; required for use in Equations 1 and 2 for the same chord are seen to be given by In order to obtain the values of the constants of proportionality, C, for the chord 3, 4-, it is necessary to set up the equations for 6 0 0 and 0 in simultaneous form, as follows:

Thus, information pertaining to all of the six points of constraint, is included in Equation 1 which is the ordinate of the curve between the points 3' and 4, referred to the chord as the X axis.

Equation 1 is derived from Equation 17 as follows:

Referring to FIG. 3, H is the departure from the chord .of the curve (a) defined by o where H is the departure from the chord of the curve (b) defined by oc The resultant curve is 

